A question from Yahoo! Answers:

What’s the purpose of screensavers?

Once upon a time, computers only had screens that showed green letters against a black background. If nothing happened on screen for a long time, the letters would develop a humongous afterglow; sometimes the screen could even be damaged by the incessant electronic bombardment. To counteract the evil effects of static content, someone came up with the idea of a screensaver; if static content hurts the screen, dynamic content will save it…

These days, screensavers can be password-protected, so that no one can access your computer while you are away from it…